The present invention relates to alternators, and particularly to alternators for use as generators in vehicles.
It is known that the magnitude of the current which can be supplied by such an alternator when the speed of rotation of the engine, and hence of the rotor, is very high, is limited by the number of turns of the winding. In other words, at high speeds of rotation, in order for an alternator to be able to supply a very high current it is necessary for the induction winding of the alternator to be made with few turns. However, this solution has the disadvantage that, at low speed, the current supply is inadvantageous. In order to obtain sufficient current at low running speeds, it would in fact be necessary for the windings to have many turns.
In order to provide motor-vehicle alternators which are capable of supplying large currents at high running speeds, and at the same time, currents of a satisfactory magnitude even at low running speeds, it has been proposed to subdivide the induction winding into two half-windings intended for connection in series with each other at low running speeds and in parallel with each other at other speeds.
FIG. 1 of the appended drawings illustrates a conventional alternator of the type described above. In this Figure two three-phase, star-connected, induction half-windings are indicated by 1 and 2. The terminals of the half-winding 1 are connected to the input terminals of a first double half-wave rectifier circuit 3, for example of the Graetz bridge type. Similarly, the terminals of the half-winding 2 are connected to the input terminals of a second rectifier circuit 4 also of the Graetz bridge type. Two diodes 5, 6 are connected in series with the rectifier circuits 3 and 4 respectively. The circuit branch comprising the rectifier circuit 3 and the diode 5, is connected in parallel with the circuit branch comprising the diode 6 and the rectifier circuit 4, between two terminals 7, 8 across which the output voltage generated by the alternator and rectified by the circuits 3, 4 appears in use. By 9 is indicated a relay comprising a movable contact member 10, and an excitation coil 11 connected to a control circuit 12. The control circuit 12 includes sensors arranged to monitor the rate of rotation of the engine, and comparator means for supplying the coil 11 with an excitation current when the rate of rotation of the engine is less than a predetermined value. In this case, the energisation of the coil causes the closure of the movable contact 10 and consequently the connection of the anode of the diode 5 to the cathode of the diode 6. In this situation, the half-windings 1, 2 are connected in series with each other through the contact 10 and the diodes 5, 6 do not carry current. A direct current which may be of considerable magnitude passes through the contact 10.
If the rate of rotation of the engine rises above a predetermined value, the control circuit 12 causes the de-energisation of the relay 9 and consequently the opening of the contact 10. In this situation the half-windings 1, 2 are connected in parallel with each other.
FIG. 2 of the appended drawings indicates the level of maximum current I which can be supplied by an alternator as a function of the rate of rotation n (revolutions per unit time). In this Figure, curve A relates to an alternator the induction winding of which is formed with many turns, while curve B relates to an alternator the induction winding of which has a smaller number of turns. For the known alternator illustrated in FIG. 1, the characteristic curve is however, represented by the first section of curve A of FIG. 2 (for values of n less than n.sub.0) and by the second section of the curve B (for values of n greater then n.sub.0). The value n.sub.0 represents the rate of rotation at which switching of the relay 9 occurs. When this relay is energised, the half-windings 1, 2 are connected in series and are equivalent to a winding formed with many turns. When the relay 9 is not energised, these half-windings are connected together in parallel and are equivalent to a single winding formed with a smaller number of turns.
The conventional alternator illustrated in FIG. 1 has several disadvantages. In the first place, the relay 9 is required to interrupt mechanically a direct current which may be rather high. This has disadvantages typical of electro-mechanical commutator devices such as, for example, the possible initiation of arcing or sticking of the contacts, a low commutation velocity etc. Such a relay cannot easily be replaced by a solid-state commutator device, for example a thyristor, since it has to cut off a direct current and not an alternating current.
In the second place, the diodes 5, 6, which are conductive when the half-windings 1, 2 are connected in parallel with each other, have to support the passage of direct currents of considerable magnitude, and have the disadvantage of a high dissipation and not insignificant cost.